Given that a storm brewed before the horn sounded, it highly likely to have been Thor himself trying to find out who killed Magni. Now I know people would say that they hate each other and Mimir mentioned only he could communicate with Jörmungandr via the Jotun language, I'm going to address two points to my hypothesis.
Even though they're enemies, that doesn't mean that you can't communicate with your enemy to gain information.
As for communication, keep in mind that Thor and many of the Norse gods have some in common, many of them have Jotun parents and in Thor's case its is mother Fjörgyn. It's highly likely that she taught him her native tongue so they can communicate with each other and he her people. Most parents from another culture will likely teach their child their language for those reasons. Another is that not only did she teach, but potentially Odin because he like Thor is half Jotun and that was how he and Fjörgyn communicated with each other.
I know there are other theories as to who could've blown the horn, Ilm just saying I wouldn't cross off Thor as a potential person to summon Jörmungandr.